The "mouse opossum" preserved in shale found in Florissant. Scale bar indicates 1 cm. (USNM-11955)
NPS*
Nanodelphys huntii
("Mouse Opossum")
Nanodelphys was a relative of the modern “mouse opossum.” The reconstruction you see here is based on a single fossil that preserves the skull, sharp teeth, rib cage, part of one forelimb, two hindlimbs, tail, and the black carbon residue of its hairy carcass. It probably fell into the lake to become preserved in the shale from its head down to its tiny fingers and toes.
Reconstruction of this "mouse opossums" skull, upper teeth per modern mouse opossums. (USNM-11955)
NPS/SIP: Mariah Slovacek
Mammal fossils are very rare in the Monument, and most of them are fragments of bone or isolated teeth. Although fossil brontotheres from Florissant were almost the size of elephants, most of the fossil mammals that have been found are very small. The majority of these are found by sieving gravel-sized sediment with screens and then sifting through the size-sorted grains under a microscope to pick out fossils from rock. However, this fossil was actually found in the lake shale.
Artist reconstruction of Florissant's "mouse opossum". (USNM-11955)
NPS/SIP: Mariah Slovacek
Like opossums today, Nanodelphys likely lived in the trees and had a prehensile tail and nimble fingers to help it climb. It's possible this one was climbing on a tree and fell off into Lake Florissant thus becoming fossilized!
Mammalian Tooth Vocab
Cusp: Pointed part’s of the chewing surface of a tooth Incisor: Front-most teeth in the mouth Canine: Single-cusp teeth Premolar: Located between the canine and molars, have two or more cusps Molar: Large teeth used for chewing, they are formed from five cusps.
The occlusal (chewing) surface of the lower first molar of Florissant's "mouse opossum," which is one of the ways scientists have classified it as unique from other fossil opossums. Inset: anterior left, lower right m1 flipped, tooth actual length 6.5 mm. (USNM-11955)
NPS/SIP: Mariah Slovacek
The bottom teeth are fairly well preserved on this individual. Per side, it had five lower incisors, a canine, three premolars, and four molars. Modern Virginia opossums have a total of fifty teeth!
Humans have two incisors, a canine, two premolars, and three molars—though most people have their third molars, called “wisdom teeth” removed, if they grow in at all!
Florissant's "mouse opossum" to scale with a hand and fossil redwood foliage. (FLFO 3661, USNM-11955)
NPS/SIP: Mariah Slovacek
Smaller than it Appears
If you could reach back in time and grab a redwood branch a Nanodelphys had perched upon, you might be surprised how small it was. Today its relatives, colloquially called "mouse opossums," still live in the Neotropics of Central and South America. "Mouse opossums," as the name implies, are far smaller than the famous Virginia opossum. The modern Virginia opossum is famous as it has led to the phrase “playing opossum” when they get scared. Opossums became extinct in North America during the Miocene epoch, but the ancestor of the modern Virginia opossum later returned when the Central American land bridge formed.
